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Search: id:A065087
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| 0, 0, 3, 12, 90, 660, 5565, 51912, 533988, 6007320, 73422855, 969181620, 13744757598, 208462156812, 3367465610145, 57727981888080, 1046800738237320, 20020064118788592, 402756584036805963, 8502638996332570140, 187953072550509445410, 4341715975916768188740
(list; graph; listen)
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OFFSET
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0,3
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COMMENT
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a(n)=(n/2)*A000240(n). - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 18 2007
a(n) is also the number of permutations of [2n-1] having n-1 isolated fixed points (i.e. adjacent entries are not fixed points). Example: a(2)=3 because we have 132, 213, and 321. [From Emeric Deutsch (deutsch(AT)duke.poly.edu), Apr 18 2009]
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MAPLE
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a:=n->sum((n+1)!*sum((-1)^k/k!/2, j=1..n), k=0..n): seq(a(n), n=0..21); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 17 2007
a:=n->sum((n+1)!*sum((-1)^k/k!/2!, j=1..n), k=0..n): seq(a(n), n=0..21); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), May 25 2007
a:=n->sum(n!*sum((-1)^k/k!, j=0..n), k=0..n): seq(a(n)*(n/2), n=0..21); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Dec 18 2007
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CROSSREFS
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Cf. A000387.
Equals 3 * A000313(n+2).
Cf. A000240.
Sequence in context: A074505 A039305 A124191 this_sequence A058337 A025503 A078124
Adjacent sequences: A065084 A065085 A065086 this_sequence A065088 A065089 A065090
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com), Nov 10 2001
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